1. Field of the Invention
The present invention relates to refrigeration circuits. More specifically, the present invention concerns the utilization of resetable pressure sensing switches to effectively regulate the capacity of a variable capacity compressor within a refrigeration circuit.
2. Prior Art
To effectively utilize an air conditioning system it is desirable to match the compressor output to the load on the system. Matching compressor output to the load on the system has been accomplished in many ways. One way is to operate the compressor motor at separate speeds thereby pumping separate amounts of refrigerant at each speed. Another way is to use valve unloaders and bypass means to limit the number of cylinders effectively pumping refrigerant within the compressor. A hot gas bypass wherein some of the discharge gas is circulated back to the compressor suction is another method of limiting compressor output. In centrifugal compressors, guide vanes are utilized to control the flow of refrigerant gas into the compressor to regulate the output by controlling the input.
The present invention is particularly concerned with a reciprocating type compressor capable of having varying refrigerant outputs in discrete stages. These outputs are controlled via unloader valves which effectively operate to render inoperative, in terms of pumping refrigerant, at least one of a pair of reciprocating pistons. To more effectively regulate the flow of refrigerant from the compressor, these individual pistons may be chosen to have varying displacements such that rendering one inoperative reduces refrigerant flow by a substantially different amount then rendering the other inoperative. Via this arrangement, a compressor having three capacity steps may be achieved by having two varying sized pistons. For a complete description of such a compressor and the control system therefor, please see U.S. patent application, Ser. No. 479,044, entitled "Variable Volume Compressor And Method Of Operating", filed Mar. 25, 1983.
In split system air conditioning units, the compressor and condenser are typically located remote from the indoor heat exchanger. In such a system it would be advantageous, in terms of energy consumption, to have a multiple capacity compressor. In split systems having multiple indoor heat exchangers serviced by a single compressor and a single condenser, the advantages of utilizing a variable capacity compressor are further increased. Such a system might typically include three indoor heat exchangers connected to a single compressor and a single condenser. The number of operating stages of the compressor could be matched to the number of indoor heat exchangers such that the load on the system may be balanced simply by selecting the appropriate stage of the compressor for the number of heat exchangers being operated. Such a system, however, is overly simplistic and, depending upon the various operating conditions of the separate indoor heat exchangers, may result in the compressor working too hard and wasting energy or being at a capacity stage which is sufficient to meet the load on just a partial number of indoor coils. For instance, should the outdoor ambient temperature be extremely high and only two indoor coils be calling for cooling (the third being shut down because the space is not being utilized) the compressor may need to operate in its highest capacity step as opposed to a lower capacity step to satisfy the load on just two indoor coils.
On the other hand, should the outdoor ambient temperature be relatively low and all three indoor fan coils are calling for cooling because of humidity conditions of the spaces being occupied, then the operation of the compressor at its highest capacity step may not be required to meet the cooling load.
The current device as disclosed herein utilizes capacity pressure sensors to determine when pressure levels have been reached. Specifically, a heating capacity pressure sensor is utilized and is connected to the compressor discharge line to sense the discharge pressure from the compressor. The heating capacity pressure sensor uses a switch arranged to move from a first state to a second state upon the pressure level being sensed exceeding a predetermined value. Hence, when the compressor discharge pressure exceeds the predetermined level of the heating capacity pressure sensor, the sensor changes from a first state to a second state indicating a need to reduce the compressor capacity. To reset the heating capacity pressure sensor, the sensor is subjected to low pressure to change the sensor from the second state back to the first state. The sensor is now in position to detect another variation above the preset pressure level. Between the heating capacity compressor sensor tripping and before the pressure sensor is again connected to sense the discharge pressure, the capacity of the compressor is reduced. As outlined in this herein application, a three state or three capacity step compressor is disclosed. If the compressor is operating at high capacity and the heating capacity pressure sensor indicates too much capacity is present, the compressor will be cycled to the next lower or midlevel capacity.
With the compressor operating in the midcapacity step should the heating capacity pressure sensor again detect a pressure level above the preset level then the sensor will again trip and the compressor will be cycled to the low capacity step.
A cooling capacity pressure sensor may also be utilized being set to trip upon the suction pressure to the compressor falling below a predetermined level. This sensor works similarly to the heating capacity pressure sensor in that upon the pressure falling below the predetermined level it changes from a first state to a second state. The capacity step at which the compressor is operated is decreased in response to the sensor tripping and the sensor is then reset by exposing the sensor to the relatively high discharge pressure from the compressor for a short interval. The cycle is then begun again with the compressor at the midcapacity stage. Should another pressure drop below the predetermined level be detected, the cycle will commence again and the compressor will then be operated in the low capacity stage.
A single control valve connected to the compressor suction line and discharge line as well as being connected by a sensing conduit to both the heating capacity pressure sensor and the cooling capacity pressure sensor acts to cycle the appropriate pressures therebetween. The valve is arranged to connect the two pressure sensors to either the relatively high pressure compressor discharge line or the relatively low pressure compressor suction line. One advantage of the herein system is that by utilizing a single control valve, only two pressure sensors, one for heating and one for cooling, are necessary. It is extremely difficult to calibrate pressure sensors at slightly different pressure levels such that the use of a series of pressure sensors staged separately from one another to control the various capacity stages of the compressor is unobtainable at a commercially acceptable expense. Additionally, the use of multiple pressure sensors is expensive and can create numerous calibration problems. The herein system utilizes a single control valve to appropriately connect pressures to both the sensors. The single control valve additionally acts to reset the pressure sensors by communicating the appropriate discharge or suction pressures to the valve to effect reset. Additionally, since the two sensors operate in distinct pressure level regions, one at high pressure and one at low pressure, a single connecting line serves both, one being rendered essentially ineffective by the pressure within the system being in the pressure range detected by the other sensor and outside the pressure range detected by the ineffective sensor. In addition, the electronic control is programmed to only sense signals from the appropriate pressure sensor for the mode of operation of the system.